imaherThe CR123A and 18650 are fancy, expensive, high-capacity batteries. The CR123A is half the height of the 18650, hence needing two of the former or one of the latter. I bought EagleTacs to go with another flashlight drop, and I've been quite happy with them. Rechargable Li-Ion batteries with 3400mAh charge: http://smile.amazon.com/dp/B00AYLA5UU/. You also need a charger for them: http://www.amazon.com/dp/B0096U26QQ/. Expensive, but definitely worth the cost in my experience.
imaherThe CR123A is a primary non-rechargeable battery. There is a rechargeable version called the RCR123A or 16340. The 18650 is a rechargeable battery. Both the RCR123A and 18650 are li-ion chemistry batteries that need a li-ion battery charger.